1. Field of the Invention
The present invention relates to a composite probe apparatus available for an ultrasonic detection of flaws or cracks being internal defects of a steel plate or a steel pipe.
2. Description of the Related Art
FIG. 4 is an explanatory illustration of an ultrasonic flaw detector using a prior composite vertical probe disclosed by the Japanese Examined Patent Publication No. 58-33501. In FIG. 4, character T represents a pulse transmitter, character PT designates a transmission oscillator, characters PR1 to PR3 denote reception oscillators, character DT depicts a transmission delaying member, character DR signifies a reception delaying member, character SP indicates a transmission and reception ultrasonic wave separating plate, character TP shows a material under examination, character RS stands for a reception input switch, character R means a reception amplifier, character F shows an internal defect of the material TP under examination, character l T indicates a dimension of the transmission oscillator, a dotted line l designates a defective ultrasonic echo path propagating from the transmission oscillator PT to the reception oscillators PR1 to PR3.
In the prior ultrasonic flaw detector, a plurality of reception oscillators PR1 to PR3 are disposed in parallel to the one transmission oscillator PT, and the transmission oscillator PT generates periodic ultrasonic attenuating vibration in response to the supply of a periodic pulse electric output from the pulse transmitter T. A transmission ultrasonic beam propagating in the material TP under examination covers a range (width) slightly smaller than the transmission oscillator dimension T and advances through the transmission delaying member DT into the material TP under examination as indicated by the dotted line l. Thereafter, the transmission ultrasonic beam is reflected at the internal defect F of the material TP under examination to proceed through the reception delaying member DR to the reception oscillators PR1 to PR3. Moreover, the transmission ultrasonic beams received by the reception oscillators PR1 to PR3 are piezoelectric-transduced into electric signals which in turn, are received by the reception amplifier R after passing through the reception input switch RS, thereby measuring the defect dimension on the basis of the echo level corresponding to the defect F.
In the case of this prior flaw detector, one of the reception oscillators PR1 to PR3, which is positioned to assume the shortest propagation distance from the position of the defect F in the oscillator width direction, receives the defect echo level with a high sensitivity. However, when the defect F lies between the reception oscillators PR1 and PR2, the echo from the defect F is dispersedly received by the reception oscillators PR1 and PR2, and hence the reception sensitivity lowers so that difficulty is encountered to ensure uniform sensitivity among the composite oscillators.
One solution to this problem is that the two successive reception oscillators PR1 and PR2 are simultaneously turned on through the reception input switch RS to receive the echo in parallel. However, the reception oscillator width is doubled to cause the sensitivity to lower and the defect detecting ability lowers because of the increase in the circuit loss of the reception input switch PR. Furthermore, according to this method, when for example the oscillators PR2 and PR3 receive the echo during the reception period of the oscillators PR1 and PR2, the middle oscillator PR2 comes into an overlapped condition so that the uniform sensitivity performance is unobtainable. For this reason, two kinds of signals are received in a time-sharing mode. On the other hand, in the case of an automatic flaw detecting apparatus in an iron and steel line where the material TP under examination moves at a high speed, accompanying the lengthening of the defect F echo reception period, this method signifies making the defect F data obtaining pitch rough or making the scanning speed low, thus lowering the defect detection performance, giving rise to great scattering of the defect detection sensitivity and impairing of the processing ability. That is, since difficulty is experienced in ensuring a uniform sensitivity and a wide effective beam width with one composite probe, the prior method requires a plurality of probes in order to ensure a wide beam width.
The plurality of probes can not be disposed on the same line in the beam width direction because of the restriction of dimension and hence they are required to be disposed in a zigzag fashion such that the separations between the probes in the direction of the transmission and reception oscillators exceed the dimension of the probes. However, this is accompanied by the increase in the dimension and weight of the probe holding device, and in the case of an apparatus such as an iron and steel line thick-plate automatic flaw detector which searches for flaws while the probe holding device follows the vertical movements of the material TP under examination, a problem exists in that the zigzag-disposed probes defy complete scanning of the material TP under examination, with the result that the range of the non-detected plate end portions enlarges, and the probe scanning device becomes complicated with increased cost.